The present invention relates, in general, to television systems and, more particularly to, a picture-in-picture processing circuit for a television system.
A picture-in-picture (PIP) feature in televisions allows the viewer to see two or more channels simultaneously on the screen. The viewer selects a main primary channel to cover the majority of the screen and alternate channels which are inserted in small portions of the screen. The viewer may thus keep track of several channels at the same time.
The television typically includes a main signal processing path for the incoming video of the primary channel. To generate the PIP images, the other video channels are converted to a digital format and stored in RAM. The data for the PIP images is averaged or compressed to fit in a smaller area of the television screen. Once a complete image of the alternate channel is stored, it is off loaded from the RAM to a digital-to-analog converter. At the appropriate point in the horizontal sweep, a controller switches from the primary channel to the PIP-constructed secondary channel to insert the PIP image. The controller then switches back to the primary channel. The result is reduced size PIP images of the alternate channels inserted in small areas of the television screen with the primary channel filling the remaining areas of the screen.
A video signal contains different types of information which is processed by the receiver section of the television. Luminance and chroma information reside in the video signal. The luma data is amplitude modulated, where the higher amplitudes are white and lower amplitudes are black. The chroma data is superimposed as a 3.58 MHz sub-carrier on the luma signal. The chroma signal contains two chrominance components B-Y and R-Y modulated in quadrature on the sub-carrier. The amplitude and phase of the demodulated chroma signal relative to a 3.58 MHz reference burst introduced at the beginning of each horizontal scan determines the saturation and hue respectively.
The chroma components of the secondary channel are stored in RAM at a normalized level. To assure proper color rendition of the reconstructed secondary signal, the reconstruction must be done with reference to the phase and amplitude of the primary channel burst signal. Feed forward gain and phase control is one approach that has been used, but has performance limitations. Using feed forward usually requires factory adjustments to trim out error. For example, in prior art systems, the hue is matched by burst locking the reconstruction circuits to the primary channel burst; due to uncertain phase delays through this circuit, a factory adjustment is required. These uncertain phase delays take place at several points; static phase error in the phase detector, phase shifts in filters and amplifiers processing the signal, and propagation delays and rise times from the clock input to the D/A Converter output. For saturation or amplitude control, the primary burst amplitude is detected to produce a DC control signal proportional to the burst amplitude. This control signal then either modifies the level of the color difference signals going to the A/D Converter and on to the RAM, or modifies the gain of a D/A Converter reconstructing the chroma signal. Errors occur in this signal processing due to nonlinearity and sensitivity of the level detector and control mechanism. Trim adjustments are subject to initial setting errors as well as drift. Blanking level errors can occur due to clamping the primary and reconstructed secondary video to different levels before combining them together. Blanking errors also occur due to different offsets in amplifiers between the clamping point and the switching point. Furthermore, blanking level match is typically untrimmed. The quality of the blanking level match is typically a function of the circuitry used.
It would be of great benefit if a circuit could be developed which precisely compensated for the differences of two video signals thereby allowing one to be inserted with the other with minimal error. The circuit would eliminate the need for factory trimming and the manufacturing costs associated thereto and compensate for the effects of aging and temperature .